sealing porous sukfaces



United States Patent T he present invention relates to methods forpreventing fluid flow through porous media and more particularly relatesto a process for coating porous surfaces on which polyvalcnt cations arepresent in order to render them impermeable. In a preferred embodiment,the invention relates to an improved core drilling process during whichthe core is continuously coated with a film impervious to water, oil andgas in order to prevent the escape of fluids originally present in theformation.

Studies of cores brought to the earths surface as a result of coredrilling operations yield much useful information concerning subsurfacestrata and the fluids contained in such strata. Porosity, permeabilityand similar formation characteristics otherwise dificult to evaluate canbe measured directly. Oil, gas and water recovered from the cores can bemeasured and analyzed. The information thus obtained provides a basisfor predicting the productive life of oil and gas reservoirs,facilitates the selection of primary recovery techniques most suitablefor particular reservoirs, and later permits the susceptibility of suchreservoirs to various secondary and tertiary recovery processes to beassessed. Experience has shown, however, that the results of suchstudies are not wholly reliable from the standpoint of the amount offluids present in the reservoir and generally must be adjusted tocompensate for fluid losses. These losses are primarily attributable tothe methods used to cut and recover cores from subterranean formations.

Conventional core drilling operations are carried out with an annularbit and core barrel rotated from the earths surface by means of a rotarydrill string. A coring fluid is circulated downwardly through passagesin the drill string, barrel and bit in order to maintain pressure on theformation. Cuttings produced by the bit are entrained in the fluid andreturned to the surface through the annulus surrounding the drillstring. As the bit cuts away the formation, the central core left by thebit is encased in the barrel. The barrel is generally provided withmeans for breaking off the core after it has been filled and withclosures which protect the core against contarm'nation by fluids presentin the borehole. Pressure core barrels which can be sealed againstchanges in pressure are also used. After the core has been cut, thedrill string is Withdrawn from the hole and the core is recovered.

Studies have shown that the pressure maintained at the bottom of theborehole during a coring operation has a profound effect upon the fluidscontent of cores subsequently recovered. If this pressure is less thanthe formation pressure, fluids contained in the formation will tend toflow out of the core into the borehole until equilibrium is established.If, on the other hand, the bottomhole pressure exceeds the formationpressure, the coring fluid will tend to flow into the interstices of theformation and displace any oil, gas or Water contained therein. Ineither case, the result is a change in the fluids content of the coresuch that subsequent measurement of the amount of fluids present willnot accurately reflect the amount originally present in the formation.Since this change occurs continuously as the core is cut, the use of apressure core barrel does not prevent it.

Several methods for avoiding the difliculty outlined above have beenproposed in the past. The most obvious of these involves carrying outthe coring operation with olefinically-unsaturated monomers.

no pressure differential between the coring fluid and the formation.This is impractical because the formation pressure cannot beconveniently measured during core drilling and, even if it could,because the fluid pressure cannot be controlled with the necessaryprecision. The use of coring fluids which will not invade the formationunder pressures well in excess of the formation pressure has beensuggested but efforts to develop a satisfactory fluid have beenunsuccessful. Mercury, plastics, molten metals and other materialsadvocated in the past all invade the formation to an appreciable extentand hence lead to changes in fluids content. In addition, the materialsproposed have generally been costly and difficult to use and in manycases required highly specialized coring bits and core barrels. Othersystems, including the use of tracers to permit determination of theextent to which core invasion occurs, have been proposed but have notbeen found effective.

The present invention provides an improved core drilling system whichobviates the difliculties outlined above and permits the recovery ofcore samples having fluid contents much closer those of the coredformations than has been possible heretofore. In accordance with theinvention, it has now been found that certain unstable polymeric laticescan be used to form an impermeable film on the surface of cores as theyare cut, thus sealing connate fluids within the cores and preventingtheir es cape. The latices do not invade the cores to a significantextent and hence do not displace fluids therefrom in significantquantities. On recovery of the sealed cores, the film can readily beremoved from the core surface. Little change in core permeability due todeposition and subsequent removal of the film occurs.

In addition to being useful as coring fluids, the latices may beemployed in accordance with the invention for a number of otherapplications. They may be used as lost circulation fluids for sealingthe walls of boreholes to prevent subsequent loss of drilling fluid toporous strata. They may be used to coat strata in order to prevent theirbreakdown due to hydration. They may be employed prior to well cementingoperations to prevent the cement from penetrating into porous Zones.They are useful as fracturing fluids in situations where low fluidlosses and non-permanent plugging are desired. They may be used to linemud pits, storage caverns and similar cavities in order to improve theirability to retain fluids. They may be used as Water-shutoff agentsduring air drilling operations. Other uses will readily suggestthemselves to those skilled in the art.

The polymeric latices utilized for purposes of the invention areemulsions of oil-resistant elastomers which readily coagulate in thepresence of polyvalent cations. When contacted with porous surfaces uponwhich polyvalent cations are present, such latices form films which areimpervious to oil, gas, Water and other fluids. Suitable latices may bederived from natural rubber or from synthetic elastomers prepared by thepolymerization of unsaturated monomers. Studies have indicated that thechemical compositions of the elastomers contained in the laticesgenerally have little effect upon their suitability for purposes of theinvention and that latices containing a wide variety of syntheticelastomers may be successfully employed.

Latices suitable for purposes of the invention may be derived fromsynthetic elastomers prepared by the polymerization ofolefinically-unsaturated hydrocarbons or by the copolymerization of suchhydrocarbons with other The olefinically-unsaturated hydrocarbonsutilized may be olefins such as isobutylene and the pentylenes;diolefins such as butadiene, isoprene, piperylene, dimethyl butadieneand 2- methyl pentadiene; or vinyl aromatics such as styrene, methylstyrene and vinyl toluene. Mixtures of such hydrocarbons may also beused. Olefinically-unsaturated monomers which may be copolymerized withthe hydrocarbons include halogenated olefinically-unsaturated compoundssuch as vinyl chloride, allyl chloride and chloroprene; unsaturatedesters such as vinyl acetate, allyl propionate, methy methacrylate,ethyl acrylate, methyl fumarate, ethyl maleate and propyl itaconate;unsaturated nitriles such as acrylonitrile, methacrylonitrile, ethylacrylonitrile and chloroacrylonitrile; unsaturated ketones such asmethyl vinyl ketone; cyclic vinyl compounds such as vinyl pyridine; andmixtures thereof.

Specific examples of elastomers prepared from the foregoing monomerssuitable in the form of latices for purposes of the invention includepolyisobutylene, polystyrene, polybutadiene, polyisoprene,butadiene-isoprene copolymers, isoprene-isobutylene copolymers,isobutylenestyrene copolymers, piperylene-vinyl acetate copolymers,butadiene-styrene-vinyl chloride copolymers, butadieneacrylonitrilecopolymers, butadiene-methacrylonitrile copolymers, andisoprene-chloroprene-vinyl acetate copolymers.

Latices containing the foregoing elastomers may be prepared by theemulsion polymerization of suitable monomers or by the emulsification oforganic solvent solutions of dry elastomers with water or other liquid,followed by removal of the solvent. The method utilized will dependprimarily upon the elastomer used. Many conjugated diolefin polymers andcopolymers of conjugated diolefins with monomers containing a vinylidenelinkage, polybutadiene and copolymers of 1,3-butadiene with styrene,acrylonitrile or vinyl chloride for example, can readily be prepared byemulsion polymerization and can be recovered in latex form. Otherelastomers, styrene-isobutylene and isobutylene-isoprene copolymers forexample, are best prepared by bulk or solution polymerization processeswhich do not result in the formation of latices. Elastomers prepared inthe latter manner must subsequently be emulsified with the aid of asolvent to produce latices. Processes for preparing latices by bothmethods are widely described in the chemical and patent literature. Atypical emulsion polymerization process is described in US. Patent2,460,038, issued to George E. Serniuk on January 25, 1949. Adescription of one method for preparing latices from dry elastomersutilizing an organic solvent may be found in US. Patent 2,799,662,issued to John L. Ernst et al. on July 16, 1957.

Latices consisting of aqueous emulsions of suitable elastomers arenormally employed in the practice of the invention but in some casesemulsions in which the con tinuous phase is a liquid other than watermay be preferred. One instance of this occurs in coring operationscarried out in very high temperature strata where a liquid having aboiling point above that of water must be used. Another instance occursin the case of arctic operations where an aqueous emulsion would quicklyfreeze during storage. Any of a number of chemically nonreactive liquidswhich have the proper temperature characteristics and do not act assolvents for the elastomers employed may be used. Normal decane, forexample, will be suitable for use with certain elastomers. Nonaqueouslatices are available commercially and methods for their preparation areknown to those skilled in the art.

The latices employed in accordance with the present invention arepreferably homopolymers prepared by the emulsion polymerization of aconjugated diolefin containing from 4 to 6 carbon atoms per molecule orcopolymers obtained by the copoly-merization of such a diolefin with oneor more olefinically-unsaturated monomers containing from 4 to 8 carbonatoms per molecule. Preferred latices include polybutadiene,polyisoprene, butadiene-styrene copolymer, butadiene-acrylonitrilecopolymer, butadienemethacrylonitrile copolyrner, butadiene-vinylacetate copolymer and but-adiene-isoprene copolymer latices.Butadiene-acrylonitrile copolyrner latices are particularly preferrcd.

The latices useful for purposes of the invention are characterized bythe fact that they are rapidly coagulated upon contact with calcium,magnesium and other polyvalent cations found in subterranean formations.They thus differ from latices employed in the past as additives forimproving the viscosit gel strength and fluid loss characteristics ofdrilling muds. Conventional drilling muds generally contain clays,barytes, gypsum, lime and similar materials in relatively highconcentrations and hence are rich in polyvalent cations. Latices used asadditives in this manner must therefore be highly stable in the presenceof polyvalent cations in order to avoid coagulation and loss of theimproved properties which they are intended to confer. Such latices aregenerally stabilized by the addition of stabilizing agents after theyare formed. They do not coagulate during use. The invention does notcontemplate the use of stable latices as additives to fluids containingpolyvalent cations and instead is predicated upon the use of unstablelatices substantially unadulterated by the presence of other materials.These unstable latices do not come into contact with appreciablequantities of polyvalent cations until they contact subsurface strata.Polyvalent cations are always present on the surfaces of such strata,although sometimes in low concentrations, and hence the laticescoagulate on contact and form impermeable films. The rapid filmformation which occurs restricts the coagulation to that part of thelatex in contact with the strata and prevents the entire latex streamfrom setting up as a solid.

Although the latices employed in accordance with the invention arerelatively unstable and quickly coagulate in the presence of polyvalentcations in very low concentrations, they should nevertheless havesufficient shear stability to permit them to be pumped without fear ofcoagulation. This requires that the stability characteristics of thelatices be carefully controlled. Latex stability depends upon a numberof factors, including the amount of emulsifier, dispersing agent orstabilizer used in preparing the latex, the pH of the latex product, andthe particle size of the dispersed elastomer in the latex. All of thesefactors are somewhat interrelated and may be Varied widely dependingupon the particular emulsifier used, the elastomer employed, and theconditions under which the latex is prepared. it is thereforeimpractical to attempt to prescribe specific properties whichcharacterize latices suitable for purposes of the invention. Thesuitability of any particular latex can readily be determined, however,by

simply contacting it with a surface coate with a very weak solution ofpolyvalent cations and by circulating it through a laboratory pump tosimulate the shear stresses encoun tered in a well circulation system,observing in each case whether coagulation of the latex occurs.

In most cases it is preferred that the latices employed in accordancewith the invention have pH values between 8.5 and about 10, although aspointed out above latices of equal stability prepared with differentelastomers and different emulsifiers may vary in pH over a considerablerange. Studies have indicated that many latices having pH values belowabout 8.5 have insufficient shear stability to permit their circulationthrough pumps and that those whose pH exceeds about 10 are generally toostable to coagulate readily in the presence of very dilute polyvalentcation solutions.

Typical latices found suitable for purposes of the invention haveaverage elastomer particles ranging between about 0.05 and about 0.1micron in diameter. Latices having average particle sizes outside thisrange may in some cases be used, since a highly eflicient emulsifier mayresult in a latex of the proper stability containing considerably largerparticles; while on the other hand a poor emulsifier may necessitatesmaller elastomer particles. The 0.05 to 0.1 micron range is, however,generally to be preferred.

Laboratory studies have shown that the solids contents of the laticesuseful for purposes of the invention are not highly critical but thatdilution with water tends to decrease their shear stability and increasetheir resistance to coagulation in the presence of polyvalent cations.In some cases this provides a means for controlling the stabilityproperties of the latices. Latices containing from about 5 to about 70%solids are generally suitable for purposes of the invention, thosecontaining from about 20 to about 50 weight percent solids beingpreferred.

The process of the invention may be carried out with conventionalapparatus familiar to those skilled in the art. A variety ofcommercially available core bits and core barrels may be used in coredrilling operations wherein the unstable latices are employed to sealcores and prevent the loss of fluids contained therein. Diamond corebits and pressure core barrels are particularly attractive for use insuch operations. In applications of the process aside from coringoperations, conventional equipment similarly available may be utilized.A wide range of tools for contacting fluids with the walls of boreholesand with other porous surfaces are described in the literature and incatalogs available from commercial suppliers and manufacturers. Furtherdescription of the apparatus employed in the practice of the inventionis therefore unnecessary.

The nature and objects of the invention can be more fully understood byreferring to the results of experimental work carried out to test anddemonstrate the use of unstable latices as coring fluids.

The apparatus utilized in carrying out the experimental work closelyresembled that employed in full scale core drilling operations. A 6 /2inch diameter core bit and a core barrel of conventional design wereattached to a length of drill pipe and rotated with respect to a blockof porous rock measuring 12" x 12" x 24" by rotating the pipe. Theblock, bit and barrel were encased in a pressure tight chamber in orderto permit the simulation of high formation pressures. Fluid wascirculated through the drill pipe, barrel and bit from a fluid reservoirby means of a high pressure pump. Fluid containing cuttings waswithdrawn through the annulus surrounding the drill string, cuttingswere removed, and the fluid was returned to the reservoir. Facilitiesfor measuring the fluids contents of cores recovered from the corebarrel were provided. This particular method and apparatus was selectedbecause earlier studies had shown that the results obtained werecomparable to actual field tests.

In a first series of experiments, a block of Berea sandstone wasprepared by first completely dehydrating the block, hermetically sealingits surfaces and applying a high vacuum. A synthetic brine closelyresembling a typical formation Water was forced into the sealed andevacuated block. Oil was then forced in under pressure. The volumes ofbrine and oil injected into the block and the volume of brine displacedby the oil were precisely measured. The saturated block contained 65%oil and 35% brine. The oil used was a 12 centipoise white oil. The brineemployed contained 2,500 p.p.m. of calcium chloride, 1,000 p.p.m. ofmagnesium chloride, and 26,000 p.p.m. of sodium chloride and thus had anaverage polyvalent cations content. A core about 17 inches long and 2 /2inches in diameter was cut from the block using the equipment describedabove and a conventional bentonite drilling mud as the coring fluid. Themud was circulated through the system at a rate of 97 gallons perminute. Mud pressure at the bit was maintained at 35 pounds per squareinch, 30 pounds per square inch in excess of the pressure Within theblock. Upon recovery of the core, it was found that the bentonite mudhad invaded it and had largely displaced the oil and water originallypresent therein. Displacement of volume percent of the connate fluids isconsidered to be the maximum displacement that can be tolerated ifanalysis of the fluids recovered from cores is to be significant.Screening tests showed that considerably more than 20 volume percent ofthe fluids originally present had been displaced by the bentonite mudand therefore further measurements of the displacement were not made.The results thus obtained are typical of field core drilling operations,where 50 volume percent or more of the oil and water contained in thecored strata is generally displaced by the coring fluid.

Additional tests using bentonite drilling muds at pressure difierentialsranging from about 40 to about pounds per square inch were made and inevery case the cores recovered had been invaded by the mud so that morethan 20 volume percent of the fluids originally present therein weredisplaced.

Following the tests using bentonite drilling muds as the coring fluid, atest was run in which an unstable latex was employed. The latex used wasone prepared by the emulsion polymerization of 65 Wt. percent butadieneand 35 wt. percent of acrylonitrile in the presence of a persulfatecatalyst and about 8 wt. percent of a potassium rosin soap emulsifier.The latex contained about 8.0 Wt. percent of an antioxidant but had notbeen chemically stabilized. It had a 40 wt. percent solids content, anaverage elastomer particle size of 0.07 micron, and a pH of about 9.5.The surface tension was 50 dynes per centimeter at 25 C. This latex wasemployed as the coring fluid during the cutting of a core 17 inches longand 2 /2 inches in diameter from a block of Berea sand stone identicalto that used in the previous tests. The sandstone was saturated with 65vol. percent oil and 35 vol. percent brine as in the earlier tests. Thecoring fluid pressure at the bit exceeded the simulated formationpressure by 50 pounds per square inch. A fluid circulation rate of 13gallons per minute was used. Analysis of the recovered core showed thatthe latex had formed a film impermeable to oil, gas and water on thecore surfam and that no appreciable invasion of the core occurred. Only4.6 vol. percent of the fluids initiallly contained in the coredsandstone were lost from the core. The film formed was easily removedfrom the recovered core. Tests showed no change in the permeability ofthe sandstone to water due to formation and subsequent removal of thefilm. No difficulties due to premature coagulation of the latex Wereencountered.

The above results demonstrate that the use of unstable latices as coringfluids permits the recovery of cores substantially unaltered withrespect to their fluids content. Cores containing fluids which arerepresentative, both qualitatively and quantitatively, of those presentin the formations from which the cores are taken can readily be obtainedby sealing the cores with such latices as they are cut. it will beapparent to those skilled in the art that the properties which renderthe latices useful as coring fluids make them equally attractive for usein a variety of other operations wherein it is desired to prevent theflow of fluids into or out of porous rock and similar earthen formationsin which polyvalent cations are present. The latices may also beemployed for sealing other porous surfaces normally free of polyvalentcations by pretreating such surfaces with a solution containingpolyvalent cations.

What is claimed is:

l. A process for recovering a core from subterranean strata on whichpolyvalent cations are present which comprises cutting the core fromsaid strata with an annular drill bit connected to the lower end of adrill string in a borehole, injecting an ion-sensitive latex into saiddrill string at the earths surface, continuously depositing a polymericfilm on the surfaces of said core as said surfaces are exposed by saidbit by discharging said latex beneath said bit in contact with saidstrata, and thereafter recovering said core from said borehole with theconnate fluids retained therein by said polymeric film.

2. A process as defined by claim 1 wherein said latex is an emulsioncontaining a butadiene-acrylonitrile c0 polymer.

3. A. process as defined by claim 1 wherein said latex contains naturalrubber.

4. A process as defined by claim 1 wherein said latex is an aqueousemulsion containing from about 5 to about 70 wt. percent of an elastomerprepared by the copolymerization of a conjugated diolefin and anolefinically unsaturated monomer.

5. A process as defined by claim 1 wherein said latex is an aqueousemulsion of a conjugated diolefin-unsaturated nitrile copolymer having apH between about 8.5 and about 10.

References Cited in the file of this patent UNITED STATES PATENTS IronsJune 21, 1938 Johnston Feb 7, 1939 Thomas Feb. 16, 1943 Golden Mar. 3,1953 Menaul Feb. 23, 1954 Davis et a1 July 8, 1958 Booth et a1 Sept. 30,1958 Williams Apr. 7, 1959 Harrison Dec. 1, 1959 Loofbourow Aug. 2, 1960

1. A PROCESS FOR RECOVERING A CORE FROM SUBTERRANEAN STRATA ON WHICHPOLYVALENT CATIONS ARE PRESENT WHICH COMPRISES CUTTING THE CORE FROMSAID STRATA WITH AN ANNULAR DRILL BIT CONNECTED TO THE LOWER END OF ADRILL STRING IN A BOREHOLE, INJECTING AN ION-SENSITIVE LATEX INTO SAIDDRILL STRING AT THE EARTH''S SURFACE, CONTINUOUSLY DEPOSITING APOLYMERIC FILM ON THE SURFACES OF SAID CORE AS SAID SURFACES ARE EXPOSEDBY SAID BIT BY DISCHAGING SAID LATEX BENEATH SAID BIT IN CONTACT WITHSAID STRATA, AND THEREAFTER RECOVERING SAID CORE FROM SAID BOREHOLE WITHTHE CONNATE FLUIDS RETAINED THEREIN BY SAID POLYMERIC FILM.